This invention resides in pay television systems including both over-the-air and cable television systems. More specifically, it resides in such systems wherein a determined characteristic of the composite television signal is changed either periodically or aperiodically at the sending end in order that a scrambled picture be received at the receiver unless a decoder is properly set up to complement the changes caused at the transmitting end and thus generate a standard television signal yielding a clear picture when applied to the conventional TV receiver.